Loss of compression

[billy sugger]

Took the 500 for its MOT today, but never reached the test centre. Bike started first time, and ticked over nicely while I finished kitting up and opened the garage. Stopped at a red light, everything still OK, but as soon as I moved off the bike stuttered like it lacked fuel, but got me to the next set of lights though still rough lights changed and as I pulled off the bike lost all engine power and began backfiring then stopped altogether. Phoned and cancelled the MOT, then called the RAC. Chap turns up in his van, I explain what's happened and his reply threw me. Because the bike had no MOT or Tax he could only arrange recovery to a garage, not my home. After a brief conversation between us he offered to help Push the bike back home, (fair play mate) which is exact what we did.

Anyway, stripped tank off to get at plugs and gave the cylinders a compression test. I have zero compression from the right hand cylinder, and I am stumped. Why has it lost compression now, yet has ran lovely whilst stationery and up and down the alley to check the gears

[iangaryprice]

Have you done any work on the engine recently? Adjusted the valve clearances?

Always worth repeating the compression check. Is there any compression at all?

Check the valves have clearance. No clearance will hold a valve open resulting in no compression.

Rotate the engine and watch the valves, do they open and close.

Unfortunately it looks like you need to remove the head to investigate what has failed.

 

 

 

 

[billy sugger]

Engine has been rebuilt after suffering a busted conrod, valve clearances were done by me in 2020,with different shims to bring back into tolerance. Replaced the big end and pistons with a complete set of crank, conrod and pistons via fleabay, all from the same year/model as mine, and from what I can find in my research there were no engine changes from 1993 until 2003 (mines 1994) so the straight swap should be fine. The only thing I had to change were the bearing journals to match the new crank. 

As already posted in a previous topic I discovered no compression in the left hand cylinder before due to the exhaust valves being slightly bent, and I straightened them out as they were only slightly bent so didn't seal properly. If the RH ones are now bent I'm flummoxed as to why

[Mississippi Bullfrog]

From how you describe the symptoms I'm wondering if the timing has slipped?

[billy sugger]

13 minutes ago, Mississippi Bullfrog said:

From how you describe the symptoms I'm wondering if the timing has slipped?

Won't know until I strip the bloody thing, but the way I'm feeling at the moment it might get left until the Xmas break 

[Old-codger]

Just a thought you said the valve/valves were bent last time so you straigthen them out. How accurate were you im thinking as the bike has warmed up a valve has stuck in the valve guide causing lack of compression.

[Mississippi Bullfrog]

2 hours ago, Old-codger said:

Just a thought you said the valve/valves were bent last time so you straigthen them out. How accurate were you im thinking as the bike has warmed up a valve has stuck in the valve guide causing lack of compression.

Isn't it the other cylinder this time?

[Old-codger]

3 hours ago, Mississippi Bullfrog said:

Isn't it the other cylinder this time?

Well spotted. Its got to be head off time then.

[billy sugger]

4 hours ago, Mississippi Bullfrog said:

Isn't it the other cylinder this time?

It is. I originally had to do the left side, but I put that down to being the side the conrod broke on, so now not so sure 

[Mississippi Bullfrog]

55 minutes ago, billy sugger said:

It is. I originally had to do the left side, but I put that down to being the side the conrod broke on, so now not so sure 

A busted conrod may have damaged the valves on the other piston as well, just not quite as noticeable at that stage.

[billy sugger]

Update. Took head off and found this. Collects were still in the collar, but the valve was stuck in the guide and no wonder. Needless to say I won't be straightening this

 

[Bianco2564]

That must have been caused by contacting the piston, assuming it was straight when you fitted it.

Cam timing incorrect?

Incorrect combination of parts, crank, rod, piston,cam?

Has the barrel or head been skimmed reducing the valve to piston clearance?

Head gasket wrong thickness?

As suggested was the valve seized in the guide?

[billy sugger]

The valve obviously sized in the guide after getting bent, but how the stem came out of the collets I don't know. Only a slight mark on the piston crown so no damage there